1. Field Of The Invention
The present invention relates to data communication and, more particularly, to the synchronization of clocks in a media data source device and an end device for displaying the media data.
2. Description Of The Related Art
Wireless networking can now advantageously provide the wireless distribution of entertainment streaming media to consumers. For example, the Sharp model LC-15L1U-S television can receive streaming audio/video streams from a DVD player using an IEEE 802.11b-based wireless network.
When media traffic is transferred over a wireless network, two different transport models can be used. In a first transport model, called a “pull” model, a receiver coupled to the end device (e.g. the television) requests data when it is needed from the source device (e.g. the DVD player). In this model, the receiver must predict when to request data from the source device such that the data will arrive, even in the event of transient transport failures and latency, in time for display. In a second transport model, called a “push” model, the transmitter coupled to the source device transmits data at a constant rate to the receiver (and thus the end device) for display.
In the push model, the rate at which the data enters the transmitter from the source device (i.e. the transmission rate) should match the rate that the data leaves the receiver to the end device (i.e. the consumption rate), with a fixed latency. Ensuring that the consumption rate substantially equals the transmission rate over a long period of time requires that the clocks of the transmitter and the receiver be synchronized. For example, in MPEG-2 based video stream transport, maintaining time synchronization between the transmitter and the receiver on the order of ±500 nanoseconds (ns) is desirable.
Problematically, drift between similarly specified system clocks could be on the order of 40 parts-per-million (ppm). For example, when a transmitter clock operating at 40 MHz is 20 ppm slow, a receiver clock also operating at 40 MHz could be 20 ppm fast. Although the IEEE 802.11 family of wireless networking standards provides a time synchronization function (TSF), the granularity of the synchronization promulgated is at best 1 microsecond (μs). Specifically, the IEEE 802.11 family of wireless networking standards calls for time synchronization to take place during beacon transmissions, which typically occur with a 100 millisecond period. As a result, using worst-case relative clock rate offsets, a 4 μs timing error between transmitter and the receiver could accumulate between beacons.
Therefore, a need arises for a method and system for more closely synchronizing the transmitter and the receiver in a digital media transport system.